London's Garden Bridge: "It feels like we're trying to pull off a crime"
It feels like that because you are trying to pull off a crime. Please hand yourself into the authorities immediately! The planning application is available online, and reveals the bridge to be a steel truss wrapped in ultra-expensive bronzed steel, rather than the pair of concrete flower-pots that I had expected. Please place some cushions on the floor around your chair before you read it.

26 June 2014

What can I possibly add to the volumes that have already been written about the Sydney Harbour Bridge?

Well, I have selected nine photos from when I visited, so here are nine thoughts (in no particular order, and not especially related to each photo!)

Completed in 1932, spanning 503m, Sydney Harbour Bridge was never the longest arch bridge in the world, but it was by far the widest and heaviest. It was pipped to the record by the Bayonne Bridge, opened in 1931, and which spans 25 inches longer. However, Sydney's bridge is probably still the most iconic of the world's long span arch bridges, situated in a beautiful harbour, and forming a key landmark visible from much of the surrounding city.

Sydney's chief engineer, J.J.C. Bradfield, had originally proposed a cantilever bridge for the site. A tour of bridges around the world convinced him that a steel arch might be more efficient, and he prepared alternative plans based on the Hell Gate Bridge in New York. Both bridges are two-pinned trussed arches, with monolithic and essentially non-structural pylons abutting the steel arch.

There are subtle differences. The Hell Gate Bridge has a more visible reverse curvature towards the upper ends of the arch, and its top chord disappears within the bridge pylons, rather than stopping short as is the case in Sydney. The upper chord carries no significant force at its ends, so the Sydney Bridge is more honest in this respect, but I think it looks quite odd.

A further difference between Hell Gate Bridge and the Sydney Harbour Bridge is in the approach spans. Hell Gate has girder spans of similar depth to the main span, with a continuity of road deck line which the pylons don't greatly interrupt. Sydney's bridge has very deep deck trusses for the approach spans, and its pylons serve a useful purpose in distracting the eye from the very different structural forms on either side.

The bridge arch is two-pinned, quite literally supported on a pin at each end. The entire weight of the bridge and the road, rail and pedestrian traffic that it carries is passed through the bottom chord into these pins. What then, is the point of the upper chord of the arch truss? The deep trusses are needed to stiffen the arch against buckling, particularly from unevenly distributed loading. However, they could converge to a point at the ends, a "crescent arch", as was the case for the much smaller Tyne Bridge, built by the same contractor, Dorman Long, in 1928.

I think the reason for the difference lies in the method of construction. Sydney Harbour Bridge was built by cantilevering from each bank of the harbour, with the steelwork tied back by massive arrays of temporary cables anchored into bedrock.

In its temporary state, the bridge doesn't behave as an arch at all, but as a cantilever bridge of the type that Bradfield eventually rejected. As such, it relies on its strength in bending to stand up, and this is why the arch truss is so deep towards its ends. It had to resist enormous bending during construction, even if this meant that the top chord at the ends became largely redundant in service. While a crescent arch might be strong enough to be built by cantilevering at smaller spans, it would simply not be deep enough and strong enough for a span of this magnitude.

The entire form of the iconic Sydney Harbour Bridge bridge is therefore essentially a relic of its process of making.

The debate on the type of arch which should have been used was rehearsed by other engineers at the time, and I'd thoroughly recommend the discussions of the papers presented to the ICE. These cover a wider variety of procurement and technical issues which remain relevant today.

Cantilever construction of large arch bridges was a common method at the time (and since), although now it would be recognised as highly inefficient (a cable-stay bridge uses its temporary erection cables as part of its permanent structure, so is almost always more efficient, as is a suspension bridge at longer spans). Indeed, other engineers commented on this at the time. David Steinman, who had submitted a design for a losing tenderer, considered that a suspension bridge should have been cheaper, and noted that Dorman Long were reported to have lost a million dollars on the contract, suggesting the arch option was under-priced.

Steinman also commented that the heavy temporary cables would have been sufficient to build a suspension bridge. Ironically, some of the temporary cables were indeed used for precisely that purpose, to build the Walter Taylor Bridge at Indooroopilly in Brisbane in 1936. Again ironically, this was one of the very few bridges in the world to copy Steinman's "Florianopolis" bridge design, an unusual trussed suspension bridge which never gained wide favour.

The design of the Sydney Harbour Bridge was controversial. A huge disagreement arose between J.J.C. Bradfield, who had conceived the bridge, and Dorman Long's designer Sir Ralph Freeman, who was responsible for the detailed design and for all the construction drawings. Bradfield claimed to be the designer, and went so far as to entirely omit any mention of Freeman in various articles, including his technical paper for the Proceedings of the ICE. Freeman, in turn, campaigned in the Sydney press for his role to be recognised, with the full support of Dorman Long. The contractor even threatened to sue the New South Wales government if Freeman's role wasn't properly credited on a plaque to be placed on the bridge.

The entire controversy is discussed in great detail in Peter Lalor's excellent book, The Bridge. In the discussion in the ICE Proceedings on the various technical papers by Bradfield and Freeman, Oscar Faber is recorded making the point that Freeman appeared to have received too little credit. Faber and several others also spoke against the design of the bridge pylons, which were regarded by many as an unnecessary extravagance.

The bridge must be one of few large bridges in the world where the public can gain access to climb the structure, albeit for a hefty fee. BridgeClimb is one of the most popular tourist highlights in Sydney, and I took advantage of it when I visited. It must make considerable profit for both the bridge owner and for BridgeClimb, as when I was there, at the height of the Australian summer, groups of visitors could be seen walking up the bridge on a very regular basis.

The BridgeClimb experience is very well organised: visitors are thoroughly briefed on what to expect, and even get to climb a trial set of staircases before being allowed onto the bridge itself. The safety arrangements are impressive, with "climbers" hooked onto a static line constantly from the beginning of their experience to the end. The scariest part of the experience is passing across the approach spans towards the pylon, where the proximity of the roadway directly below the catwalk is a little disconcerting. Once past the main pylons and onto the main span, the climb is straightforward, and not at all terrifying. The large width of the main arch chords is such that you never feel close enough to an edge to feel at all worried.

For anyone wanting to know more about the bridge, I can recommend Peter Lalor's book (see below), which is from the perspective of a journalist, concentrating on the human-interest stories, and Peter Spearritt's (also see below), from the perspective of a historian, very well illustrated and placing the bridge narrative in more of a social context.

23 June 2014

This isn't the firsttime I've featured an elevated walkway in this blog about bridges. It's not really a bridge, because it doesn't cross a particular obstacle.

Located above an escarpment some way to the south of Wollongong, New South Wales, the walkway provides views both into the rainforest canopy, and down from the escarpment across the lowlands towards the ocean. It's only part of a rainforest trail, but it stretches some 500m long, 25m above the ground. Completed in 2008, it cost about AUS$6.5m.

Despite the reported cost, it looks like it has been built on the cheap, with a mesh floor, open parapets, and somewhat industrial trusses below the walkway. It seems almost designed to enhance vertigo rather than to provide reassurance. There are two sections, one near each end of the walkway, which are cable-stayed cantilevers, and which sway a little alarmingly when loaded.

As if those weren't enough to deter the faint-hearted, a central tower rises to an even higher viewing platform, reached by a spiral staircase. When I was there, I saw at least one visitor unwilling to attempt the climb, and I didn't blame them.

Nonetheless, the walkway offers some fantastic views, not only across the landscape but also of the forest canopy, and down to the undergrowth, dominated by gigantic ferns.

19 June 2014

I spent some time in Australia a few months ago, and while I didn't get time to visit many bridges, I did see a couple which are worth featuring here.

The Sea Cliff Bridge is on the coast of New South Wales, north of Wollongong. It was built in 2005 to replace a section of highway repeatedly affected by landslides and rockfalls. The bridge is 665m long, and represents a significant engineering achievement, not only for having been built at such an exposed location.

The southernmost section of the bridge is in the form of a post-tensioned balanced cantilever bridge, with 108m main spans. This is built on a varying curve in plan, meaning that every 5m long concrete box segment was different in curvature, angles, as well as in depth, with the spans varying from 6m deep over the piers to 2.5m at midspan.

The northern section of the bridge, approximately 200m long, has much shorter spans of 30m, and a constantly curved radius in plan of 150m. The form of the deck is different, comprising twin curved concrete beams, of constant depth, matching the depth of the balanced cantilever bridge where they join so that in elevation they appear to be continuous structure. The north part of the bridge was installed by incremental launching.

The combination of the two construction methodologies is unusual, and I'm impressed by the careful construction control that will have been required.

Despite the spectacular setting, the bridge is not especially exciting to drive over, as views off the highway are largely obscured by precast concrete barriers. It is better appreciated by walking down to the shoreline below. As well as the bridge itself, there are some impressively eroded rocks to see.

17 June 2014

I've posted links below to sites where you can find more information about the entries. I've only visited two, the Media City Footbridge and the Stratford Town Centre Link. I've also featured the Jarrold Bridge here, although not visited it yet. Most of the others are new to me. Some of them are quite brilliant, so it's well worth taking time to browse through the links below.

16 June 2014

I'm preparing various new posts, but while that happens, I thought I'd post a link to a great website I stumbled across: Swiss timber bridges. Written by Werner Minder, this is a catalogue of some 1,481 timber spans in Switzerland, with plenty of pictures.

Unsurprisingly the website majors on covered wooden bridges, some 520 of them, but has space for structures as varied as suspension bridges, timber falsework, and even a tiny bridge for rabbits. It's the kind of detail-obsessed labour-of-love that I love finding on the internet.

12 June 2014

At last, this was the final bridge in the IABSE Study Tour of north-east England.

Completed in 1983 to a design by Mott, Hay and Anderson, the present Redheugh Bridge is on the site of two former crossings. The first was the most interesting: an innovative cable-stayed design by Thomas Bouch, which survived from 1871 to 1897.

Spanning 160m, the current bridge is a post-tensioned concrete box girder structure. An attractive steel box girder design had been proposed by MHA in 1966, but this was abandoned when a spate of failures of this type of bridge occurred, coinciding with a rise in steel prices which rendered a sufficiently strong design uneconomic.

The bridge has a twin-cell single box configuration, with utility services carried inside the box - gas pipes in one cell, and electricity cables and a water pipe in the other cell. Holes in the concrete box are provided for ventilation and as drainage in case the water main should leak.

The bridge was constructed using the balanced cantilever technique, with a series of 3m long in-situ concrete sections. It was completed a year after the Byker Metro Viaduct, another post-tensioned concrete balanced cantilever bridge built nearby. The Byker bridge was more innovative, being built using glued precast units, the first of its type in Britain, and I think it was also far more successful aesthetically. However, it's fair to note that the Byker bridge carries significantly lower loads, and has much shorter spans, so the pressure to adopt only the most economic solution will have been a little less.

That concludes my tour of bridges in the north east of England. I have a backlog of other bridges to report on here, and hope to start my next series of reports soon.